Approximately 113,000 bariatric surgeries were performed in the
United States in 2010; as many as 80% of persons seeking weight loss
surgery have a history of a psychiatric disorder. (1), (2)

Bariatric surgery can be "restrictive" (limiting food
intake) or "malabsorptive" (limiting food absorption). Both
types of procedures can cause significant changes in pharmacokinetics.
Bariatric surgery patients who take a psychotropic are at risk of
toxicity or relapse of their psychiatric illness because of
inappropriate formulations--immediate-release vs sustained-release--or
incomplete absorption of medications. You need to anticipate potential
pharmacokinetic alterations after bariatric surgery and make appropriate
changes to the patient's medication regimen.

Pharmacokinetic concerns

Roux-en-Y surgery is a malabsorptive procedure that causes food to
bypass the stomach, duodenum, and a variable length of jejunum.
Secondary to bypass, iron deficiency anemia is a common nutritional
complication.

Other changes that affect the pharmacokinetics of psychotropics
after bariatric surgery include:

With time, intestinal adaptation occurs to compensate for the
reduced length of the intestinal tract; this adaptation produces mucosal
hypertrophy and increases absorptive capacity. (3)

Medications to taper or avoid

The absorption and bioavailability of a medication depend on its
dissolvability; the pH of the medium; surface area for absorption; and
GI blood flow. (4) Medications that have a long absorptive
phase--namely, sustained-release, extended-release, long-acting, and
enteric-coated formulations--show compromised dissolvability and
absorption and reduced efficacy after bariatric surgery.

Avoid slow-release formulations, including ion-exchange resins with
a semipermeable membrane and those with slowly dissolving
characteristics; substitute an immediate-release formulation.

Medications that require acidic pH are incompletely absorbed
because gastric exposure is reduced.